The incident occurred at night in the vicinity of Chengdu, about 1.5h before landing, as the aircraft deviated to the right of its intended flightpath in order to avoid a thunderstorm.

At about 18:00UTC the aircraft entered a veil of cloud not displayed on the weather radar. The crew noticed a rise in air temperature but did not see evidence of ice deposit.

Flight-recorder data showed the static air temperature increased from minus 54C to minus 34C for 86s. The crew switched the engine ice-protection system from automatic to manual, and the aircraft flew for around 10min in this mode, Rosaviatsia states, before reverting to automatic.

Some 22min after the cloud encounter the aircraft's inboard left-hand engine underwent a surge and automatic restart. Two minutes later the neighbouring outboard engine experienced a speed reduction to 70% of N1, while the inboard remained at idle thrust.

Rosaviatsia says the aircraft landed at Hong Kong and inspection found damage to the first-stage high-pressure compressor blades of both left-hand engines as well as the outboard right-hand powerplant.

In a bulletin to carriers the authority says the aircraft was caught in a region of "unexpected" and "unpredictable" weather conditions, leading to ice deposit on unheated parts of the high-pressure compressor. Rosaviatsia notes that the lack of comprehensive knowledge about crystal icing, and the difficulty of detecting the phenomenon, constitutes a continuing hazard.